System for producing and distributing compressed air

ABSTRACT

A system for producing and distributing compressed air that comprises at least one compressor ( 1 ) having connected thereto a suction pipe ( 2 ) for the intake of air and an output pipe ( 3 ) for air compressed by the at least one compressor, and distribution piping ( 4 ) connected to the output pipe ( 3 ) for distributing air to sites of use ( 5, 6 ). The system further comprises a return pipe ( 7 ) arranged between the suction pipe ( 2 ) and at least one site of use ( 5 ) for receiving air reduced in pressure in it and feeding it back to the at least one compressor ( 1 ).

FIELD OF THE INVENTION

The present invention relates to a system for producing and distributingcompressed air that comprises at least one compressor having connectedthereto a suction pipe for the intake of air and an output pipe for aircompressed by said at least one compressor, and distribution pipingconnected to the output pipe for distributing air to sites of use.

The invention also relates to a system for producing and distributingcompressed air that comprises at least one compressor having connectedthereto means for suction air intake and an output channel for aircompressed by said at least one compressor, and distribution pipingconnected to the output channel for distributing air to sites of use.

The invention thus concerns industrial and instrument air systems, inwhich the conventional pressure level is 10 to 15 bar or less, in whichthe pressurized dew point of the compressed air is generally appropriatefor the intended purpose, i.e. even −40° C., and in which the length ofthe manifold and distribution piping can be several kilometers.

In a conventional compressed air system, described above, thecompressed, after-treated air discharges into the environment after use.Correspondingly, the compressors generally obtain untreated air forcompression from the environment through a suction pipe. Since suctionair contains dirt particles, it usually needs to be filtered for thefirst time already in a suction filter before it enters the compressorand is used. Filtering causes a certain negative pressure in the suctionpipe depending on the filtering fineness and the degree of filtercontamination, which in turn increases the energy requirement of thecompressor to some extent. In addition, the suction filter requiresservicing and maintenance, which causes additional costs to theproduction of compressed air. Suction air often also contains causticgas components that enter the compressor with suction air and may causecorrosion in the air compression space of the compressor, when heatingup during compression and when the concentration increases.

An especially bad situation in this respect exists in unlubricated, i.e.dry, screw and piston compressors, in which oil does not protect themfrom corrosion. To eliminate this common problem, the inner parts ofthese compressors are made of corrosion-proof materials. For instance,the screw units of an unlubricated screw compressor are coated withKevlar or some other coating or they are made of corrosion-proofmaterials. Thus, the price of these compressors is high due to the highmanufacturing costs of the screw elements, for instance.

In lubricated screw and rotary compressors, too, foreign gas componentscan end up with the circulated cooling and sealing oil, weakening itsproperties and consequently, causing deterioration in lubrication. Thisis why the oil needs to be changed relatively often. The oil isrelatively expensive, because in this task, it is required to have manyspecial properties. Due to the above matters, producing compressed aircauses considerable fixed and variable costs, when done by a screwcompressor, for instance.

Suction air always contains moisture, because air always contains watervapour. Water needs to be removed from compressed air before use. Arequirement can be that the maximum pressurized dew point is −20° C.,for instance, which means that water does not condensate in the pipingwhen the compressed air remains at a temperature above said level andthe piping does not freeze. For this purpose, compressed air systems areequipped with dewatering systems and different types of dryers toachieve the desired pressurized dew point. For the same purpose, aconsiderable number of other components are needed, such as differenttypes of water reducers, an after-cooler for lowering the temperature ofthe compressed air and different types of filters, the number of whichdepends on the compressor type, for instance. The amount of removedwater can be very large, such as 100 liters per 24 hours.

Oil-lubricated compressors, for instance, typically always have anexternal (located after the compressor package) coarse and fineoil-separating filter prior to the actual adsorption dryer. In addition,some of these compressors have internal separating filters forseparating drop and aerosol oil integrated to the compressor package.After the often-used adsorption dryer, there is also a dust separationfilter and sometimes even an active carbon filter and bacterial filter.Oil-lubricated screw compressors also have an oil trap for the purposeof separating the oil, which has ended up in the compressed air from theoil cooling of the compressor, and condensed water from each other. Thewater condensate is usually run into the sewer, even though it still atthis stage has some oil residue. Oil traps do not remove any waterpollutants possibly carried along with the suction air.

Everything described above is called after-treatment of compressed air,in which solid particles, oil and water is removed from the compressedair. The corresponding equipment is called a compressed airafter-treatment system. The most comprehensive after-treatment system isfound in the very commonly used oil-sealed screw compressor systems, inwhich the essentially most important component of the after-treatmentsystem is the dryer, but a number of filters and other equipment arealso needed for oil removal. The extent of separating these in each caseduring after-treatment depends on the required compressed air qualityclass according to the ISO 8573 standard.

The after-treatment of compressed air forms approximately 25% of theprice of compressed air. This includes fixed costs, too, but mainly thecosts are variable costs, of which the share of energy is significant.The filters, for instance, typically cause a total pressure loss of1,500 kPa depending on their degree of contamination, which means anearly 10% increase to the energy required to produce compressed air,because the compressors must operate with a delivery pressure that ishigher to the extent of this pressure loss. The after-treatmentequipment requires a great deal of servicing and maintenance, which alsoincreases the variable costs.

In lubricated compressors, especially in oil-sealed screw and rotarycompressors, the temperature of the compressed air cannot be loweredbelow a certain level, because then the moist air coming from thesuction pipe with the compressed air would condense into water in thecompressor and the oil used for cooling and sealing would be whiskedtogether with the water into an emulsion as the rotors turn. As apaste-like substance, this emulsion would cause blockage in the filters,and the oil trap would not work as desired, because oil and water form amixture that the present equipment cannot separate. In time, if nothingis done, this leads to a stoppage in the production of compressed air,which may cause a shutdown in the production of the industrial plant.

For this reason, in these compressors, the temperature of the suctionair and the output air from the compressor must be kept generally atleast at +60° C. depending on the temperature of suction air. This, inturn, results in the need to control the cooling of the compressed air,in other words, the temperature and/or volume flow of the cold coolingoil sprayed into the space between the rotors, so that the temperatureof the air to be compressed would not drop too low. This also means thatthe compression process in the compressor is isentropic with anisentropic exponent of nearly 1.3. The compression is far from an idealcompression process requiring the least amount of energy and takingplace at a constant temperature, i.e. an isothermal compression process.This means that the specific energy consumption of the compressor ishigh. The isothermal efficiency of these compressors is probably in therange of 70%, so approximately 30% more energy is consumed in thecompressor than in ideal compression at constant temperature.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a compressed airsystem, in which the above-mentioned problems are at least mainlyeliminated. This is achieved by a system for producing and distributingcompressed air according to the invention, which is characterized inthat the system further comprises a return pipe arranged between asuction pipe and at least one site of use for receiving air reduced inpressure in it and feeding it back to said at least one compressor. Oneversion of the invention is characterized in that the system comprisesreturn means for receiving air reduced in pressure in the usage site andfeeding it back to said at least one compressor.

The basic idea of the invention is thus that at least some of the airused in the system remains in the system and consequently, need not bedried after various compression times. This results in immediate costsavings. The larger the amount of system air that can be kept within thesystem, the bigger the savings are.

In the compressed air system of the invention, the dried compressed airused in the distribution system is returned to the compressor as(compressor) suction air. This air to be compressed is dried and of highquality, and the removal of the compression heat of oil-sealed andoil-cooled screw and rotary compressors can easily be improved to suchan extent that compression is done nearly isothermally in thecompressor, because the moisture of the suction air cannot now causeproblems with the compressor oil separation. This way, a saving of up to20% is achieved in the energy consumption required to produce compressedair. Other advantages include a significant improvement in oilseparation in the screw and rotary compressors that have an internal oilseparation system, because oil sprayed at a lower compressiontemperature remains in drop or aerosol format and can thus be moreeasily removed from the compressed air already inside the compressor. Inthe present compressors, in which the compression is isentropic and,therefore, the compressed air is hot, oil enters as vapour into thepiping and it is not possible to remove it completely and even reducingthe amount requires specific filtering systems.

Because the compressed air system of the invention can be completelyclosed and if there are no leaks in it, it is also possible to use othergases than outdoor air, such as dry nitrogen gas, as the medium. Allcompressor types can compress nitrogen gas. If there are leaks in thesystem, they can be easily detected and measured. Leaks can becompensated for in many ways, for instance by a separate smallcompressor producing dry air, or if there are other sources of dry air,by taking the replacement air from them. Dry air circulation is eventhen maintained in the system.

Because moisture does not enter the system with the suction air, as inconventional open compressed air systems, a sound leak-free systemrequires no after-treatment equipment.

In oil-sealed screw compressors, the oil trap also becomes unnecessary.As a result of this, the compressor can be operated using a lowerpressure, because there is no pressure loss in the after-treatmentequipment, which can in a conventional industrial air system be as highas 1,500 kPa. This means that the compressor output decreasesapproximately 10%, because the specific energy decreases strongly whenthe delivery pressure of the compressor decreases. In addition, theenergy-consuming recovery of the adsorption dryers or, in the case of acooling dryer, the electric energy required to run the coolingcompressor, is left out.

The return pipe, which can be connected to the suction air connection ofthe compressor, also does not apparently require a suction air filter toremove mechanical particles and caustic gases do not enter the systemtogether with the suction air, so the inner parts of the compressor arenot corroded. The compressors can then be inexpensive compressors withnon-corrosion-protected compression and displacement spaces. Noisetransmitted from the suction pipe to the environment is also reduced.

The suction air of a compressed air compressor is usually taken from aspace having as good quality air as possible: a minimum degree of dust,no caustic gases, no combustion engine exhaust gases, etc. The suctionpipe is at best located on a south or east facing side, where thetemperature during summer is as low as possible. These factors limit theselection of the location of the compressor or compressor centre. Nosuch limitations exist in the compressed air system of the invention,and the compressors can be located freely, for instance outdoors. Thereis no danger of malfunction even during frost, if air-operated heatexchangers are used. The pressurized dew point of the air to becompressed is then expected to be sufficiently low, in other words, theair should be so dry that water is not condensed from the compressed airand freeze even during very low below-zero temperatures.

In conventional compressed air systems, the following compressed airtreatment apparatuses are needed in the given order for instance whenthe pressurized dew point requirement is −40° C., as in pneumaticinstrumentation systems, and an oil-sealed screw compressor is used: anactual compressor unit that contains, integrated into the same package,a suction air filter, an actual pressure-generating screw unit and atwo-phase oil-separating cyclone and filter combination; a compressedair tank; an oil-separating filter; a fine oil-separating filter; anadsorption dryer; a dust filter; and sometimes also an active carbonfilter and a bacterial filter. In addition, an oil trap is also needed.The compressed air system of the present invention does not require thesuction air filter, compressed air tank or the other pressure-sidefilters, if used in the special case where the compressor is anoil-sealed screw compressor and the suction air is treated in such amanner that its pressurized dew point is sufficiently low and does notcontain mechanical impurities. The adsorption dryer and the oil trap arethen also unnecessary. Thus, all after-treatment devices areunnecessary. In addition, the compression process of the compressor canbe made nearly isothermal by improving to a sufficient extent the oilcooling directed to the air being compressed between the screw elements.This is possible, because there is no moisture in the suction air. Theinternal oil separation of the compressor package is then improved sothat practically all oil is separated, because no oil vapour isgenerated due to the low compression temperature. If for the purpose ofsaving energy, a higher than atmospheric pressure is to be used in thesuction pipe, the pressure endurance of the suction side can easily bechanged in a standard compressor and in a situation where thecirculating compressor is a booster compressor, i.e. a pressure boostcompressor.

If the system of the invention is leak-free, it is also possible toeconomically use other gases than air in it. One such gas is nitrogen.In a closed system like the one described herein, means for drying thegas are not needed. Only when the system is taken in to use, it isnecessary to use either dried air or separate means for drying the airfed into the system.

In usage sites, from which the compressed air is led into return piping,the system of the invention also makes possible a procedure, in whichafter the usage site, the air pressure is not the normal atmosphericpressure, but even significantly higher than that. This type ofcompressed air drive connected to a return cycle can thus be arranged tofirst have a 10-bar pressure and afterwards a 3-bar pressure, in whichcase the pressure difference over the drive is 7 bars. Depending on thecompressor properties, approximately 40% less energy is required toraise the air pressure from 3 bars to 10 bars than when raising thecompressed air pressure from 0 bars to 7 bars. Thus, the use of apressure level higher than the normal atmospheric pressure after theunit, and consequently in the suction pipe of the compressor, alsoreduces significantly the operating costs of the system. This ispossible, because the force of a double-acting cylinder, for instance,is the same in both cases.

If the system cannot be made completely closed due to compressed airdrives, such as blow, spray-painting or pneumatic pipelining drives, inwhich air cannot be recovered, means for replacing the removed air arerequired in the system. Such a means can be a second suction pipeconnected to said at least one compressor for feeding in replacementair, which replacement air can be either untreated moist outdoor air orair that is dried and substantially moisture-free. If moist air is used,the system needs a dryer, through which this moist replacement air isrun to achieve the desired dew point. In this case, too, only a part ofthe air in the system needs to be replaced and dried, and thus, thedrying capacity of the system can be significantly lower than inconventional compressed air systems. Replacement air intake can also bearranged to be periodical, i.e. to occur only when the pressure in thereturn pipe decreases too much. Drying then also needs to be done onlyperiodically, which leads to significant savings in the operating costs.

In another alternative, especially if the need for replacement air isgreat, the air required by the blow drives is fed with the originalcompressors having dryers. Their own closed system having a circulationcompressor can then be used for drives, in which exhaust air can berecovered. Because this second system is in the distribution piping inan area, in which air is already dry, a dryer is not needed in thissystem. The filling of the system and a possible compensation for leaksin this system can easily be done using the distribution piping of theprevious system.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the system for producing and distributing compressedair according to the invention will be described in greater detail withreference to the attached drawing, in which

FIG. 1 is a very simplified diagram of a first exemplary embodiment ofthe system of the invention, and

FIG. 2 is a very simplified diagram of a second exemplary embodiment ofthe system of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows by way of example a very simplified diagram of a firstembodiment of the system of the invention. This diagram includes onlythe components of a compressed air system that are essential for theinvention. Thus for the sake of clarity, conventional and, in part,necessary devices of compressed air systems, such as after-treatmentdevices for the air produced in the compressor, for example anafter-cooler, compressed air tank, dryer, oil separators or separatingdevices of other solid particles, are not shown.

The system shown in general in FIG. 1 comprises a compressor 1 with asuction pipe 2 and output pipe 3 connected to it. The output pipe 3 isconnected to compressed air distribution piping 4 leading to devices 5,from which output air can be recovered. From the devices 5, a returnpipe 7 leads to an air tank 8 that is, in turn, connected to the suctionpipe 2 of the compressor 1. The tank 8 is, however, not necessarilyneeded in the system, especially if the volume of the return pipe issufficient per se. If the system does not contain a drive, such as ablow drive or the like, in which air cannot be recovered, as shown by adashed line and marked with the reference numeral 6, the system can bemade fully closed. All compressed air led to sites of use 5 is thenrecovered to the return pipe 7 and can be returned to the compressor 1.This type of system can also be easily implemented in the currently usedcompressed air drives. It is, for instance, possible to guide the airdischarged from the control valves of regulating units, through whichthe air that is reduced in pressure discharges, to the return pipe of acirculating compressor. In FIG. 1, equipment 9 represents theafter-treatment equipment of compressed air with any possible dryers,through which compressed air can, if necessary, be driven in connectionwith after-treatment; it can also be by-passed, if necessary. The returnpipe 7, i.e. the means for receiving air reduced in pressure in one ormore sites of use 5 and for feeding it back to at least one compressor,thus returns compressed air to the input side of the compressor, i.e.directly or indirectly to the suction pipe. The suction pipe is themeans for bringing suction air to the compressor. In FIG. 1, the returnair in the return pipe 7 comes through the tank 8 to the suction pipe,and correspondingly, it is possible to use for instance animplementation, in which the return of the return air in the return pipeis through an intermediate pressure tank between the first and secondcompressor phase to the suction side of the second compressor phase, ifthe compressor is a two-phase compressor.

The invention can also be examined in a system for producing anddistributing compressed air that comprises at least one compressor 1 or21 with means 2 or 22 for the intake of suction air connected to it andan output channel 3 or 23 for air compressed by said at least onecompressor, and distribution piping 4 or 26 connected to the outputchannel 3 or 23 for distributing air to the sites of use 5, 6 or 25. Thesystem further comprises return means 7 or 27 for receiving air reducedin pressure in the site of use and feeding it back to said at least onecompressor 1 or 21. The means 2 or 22 for the intake of suction aircomprise a suction pipe 2 or 22 and the return means comprise a returnpipe 7 or 27.

If the system is completely closed and does not need air from theoutside, no external moisture enters the system and it does not need adryer providing sufficiently dry air is used when the system is filled.Dry-air filling can be done by a separate filling system including asuitable dryer or by using pressurized air that has been sufficientlydried to contain no moisture when produced. Even though the system ofFIG. 1 in its most conventional form contains air as pressurized gas,its closed structure also permits the use of some other gas, such asnitrogen, if this is otherwise advantageous especially due to thestructure of the drives, such as the regulating units.

Because the system of FIG. 1 does not in principle have a dryer andthere is no need for one, a significant amount, such as a quarter, ofcompressed air production costs can be saved in comparison with aconventional system that does not have circulation and in which all airused in the system always needs to be dried.

The system of FIG. 1 also makes it possible to raise the pressure levelsof the drives, if the structures of the drives are suited for the higherpressure. For instance, it is possible to use the system in such amanner that the pressure in the distribution piping after the compressoris 14 bar, for instance, and the pressure in the return pipe after thedrive is 7 bar. The power needed by the compressor of the system is thenonly approximately 30% of the power that would be needed if the systemwas used in such a manner that the pressure after the compressor was 7bars and the pressure in the return pipe was 0 bar, i.e. atmosphericpressure. The above numerical values are only an example of what raisingthe general pressure level of the system can achieve in cost saving. Itis more probable that the pressure levels must be kept lower thandescribed above especially due to the fact that conventional compressedair drives are not suited for use at the pressures described above. Inany case, the generation of a normal operating pressure difference insuch a manner that a predefined counter pressure also prevails after thedrive leads to a significant cost saving.

A dashed line in FIG. 1 shows a compressed air drive 6 that is thoughtto be a blow drive, i.e. a drive in which compressed air cannot berecovered. Therefore, air escapes from the system through it. To replacethe escaped air, the compressor 1 is equipped with a second inlet 10shown by a dashed line. If normal air, i.e. air containing moisture, istaken in through this inlet, an after-treatment equipment 9 includingdryers, which is shown by a dashed line, must be included into thesystem to remove the moisture in this replacement air. A valve 14 alsoshown by a dashed line then closes the direct pipe connector 3. Ifpre-dried air is fed to the inlet 10, the dryer is naturally not neededor it can be by-passed. In any case, the after-treatment equipment 9 canbe significantly smaller in capacity and moisture removal ability,because it only needs to dry the air required by the blow drive 6. Theafter-treatment equipment 9 can also be used in such a manner that airis run through it only when drives, which let air escape from thesystem, are in operation. The after-treatment equipment thus need not bekept in continuous use, which also saves energy. In such a case, thereis no pressure loss, because air is run past the after-treatmentequipment 9.

The blow drive 6 can alternatively be thought to represent leaks thatexist in most compressed air systems. If the system is otherwise fullyclosed, leaks in the system can be very easily and reliably detected andtheir size measured in the system of FIG. 1. Namely, if pipe leaksexist, this results in an immediate decrease in pressure on the suctionside of the compressor 1, if the compressor delivery pressure is keptconstant. This pressure decrease can be easily measured and the amountof air escaped from the system thus determined, when the combined volumeof the suction pipe 7 and the tank 8 is known. The easy measuring of apossible leakage amount or leakage flow is another a significantadvantage of the closed system of the invention over a conventional opencompressed air system.

Possible leaks can be compensated either in the manner described above,in which the compressor is equipped with a second inlet 10, or bysupplying after-treated and dry air into the distribution piping. Dashedlines 11 and 12 in FIG. 1 show this supply. The inlet 11 connects to thedistribution piping before sites of use 5 and the inlet 12, in turn,connects to the return piping 7. All above-mentioned replacement airsupply routes 10, 11 and 12 are also connected to a unit 13, which is agauge that measures the air volume flow and/or amount of air flownthrough the unit and thus provides direct information on the leakageflow of the system or its amount or the volume flow or amount of airthat the leaks and drives, in which air cannot be recovered, consumetogether. The unit 13 can also contain a check valve, pressure-reducingvalve or pressure-regulating valve, by means of which the connection toan external air source can be opened or closed as desired or replacementair can be automatically let in to the system, if necessary. Tocompensate for leaks, the system should be equipped with one of thealternatives shown by dashed lines in FIG. 1. When using the inlet 10,the inlet 2 is closed with a valve 50, for instance.

The above describes, how leaks can be measured in a closed system. Thesame principle can also be used to measure the compressed air amount orvolume flow consumed together by the blow drives and possible leaks. Ifthe volume flow required by the blow drives is known—this being easilydetermined by commercial volume flow sensors—piping leak flow can beobtained from the total volume flow by subtracting the volume flowrequired by the blow drives. Correspondingly, if it is known that thereare no piping leaks, the method can be used to determine the compressedair volume flow or amount used by the drives, in which air cannot berecovered.

FIG. 2 shows by way of example a very general diagram of a secondembodiment of the system of the invention. In it, the sites of use ofthe system are marked with the reference numerals 25. In these sites,all air can be recovered. In this system of FIG. 2, a system thatessentially corresponds to that of FIG. 1 is build around the drives 25enabling air recovery. The system comprises at least one compressor 21that has a suction pipe 22 and output pipe 23 feeding compressed airthrough distribution piping 26 to the drives 25, and a return pipe 27that connects the drives to the suction pipe 22 of the compressor 21.The suction pipe 22 is connected to the compressed air distributionpiping 20 of an industrial plant, for instance, by means of equipment 24that contains at least a valve, which is possibly a pressure-reducingvalve or pressure-regulating valve, through which additional air isreleased into the suction pipe 22 when necessary.

The compressed air distribution piping 20 belongs to a compressed airproduction system that, when necessary, provides sufficientlyhigh-pressure, such as 8-bar, after-treated compressed air having asufficiently low dew point. By means of the equipment 24, thiscompressed air can be released into the suction pipe 22 of thecompressor 21 either for filling the system or for compensating forpossible leaks. The equipment 24 can thus contain a check valve,pressure-reducing valve or pressure-regulating valve, by means of whichthe pressure level of the suction pipe 22 can be set to 2 bars, forinstance. The equipment 24 can also contain a flow meter, by means ofwhich the need for replacement air, i.e. the amount of leaks in theclosed circuit containing the compressor 21, is directly revealed. Ifthe pressure of the suction pipe 22 is 2 bars, as mentioned above, andthe delivery pressure of the compressor 21 is 9 bars, a pressure of over7 bars affects over the sites of use 25.

Because the air in the piping 20 is already dry, no separate dryer isrequired in this closed branch connected through the equipment 24, andair coming from the compressor 21 can be directly fed to the drives 25enabling air recovery. Returning the dry air reduced in pressure throughthe return piping 27 to the suction pipe 22 leads to not needing tore-dry the air circulated through the drives 25. The system of FIG. 2thus achieves the same savings by means of air recovery from the drives25 as described earlier in connection with the system of FIG. 1.

The system of FIG. 2 also has components, connected to it by dashedlines, that relate to a situation, in which the compressor 21 does notfor some reason produce compressed air. Safety arrangements arenecessary, if the continuous operation of the drives 25 is to besecured. This has to do with a so-called primary network implementationfor important sites of use 25. The availability of compressed air forthe sites of use 25 is then secured even in a situation, in which thecompressor 21 cannot produce compressed air. The option of a primarynetwork solution is another advantage of the closed system of theinvention over the conventional open compressed air system. If thecompressor 21 is damaged, the pressure of the network 20 is let directlyinto the output pipe 23 of the compressor by by-passing the compressor21 by means of a by-pass line 28 and a valve 30 in it and by controllingthe equipment 24 in such a manner that a direct connection is opened tothe industrial air network 20. Because the circulation compressor is notoperating, air that is reduced in pressure is let out after the drives25 through an output 29 by opening its valve 32. In addition, the outputof the equipment 24 needs to be disconnected from the return line 27 byclosing a valve 31 in it so as to prevent the pressure of the network 20from discharging through the output 29. The system is then open, becauseair from the drives is not recovered for circulation, but directedoutside using the pipeline 29 and valve 32. The pressure of thecompressed air network 20 then acts on the compressed air drive 25, sono interruption in use occurs.

The system of FIG. 2 is also interesting in that it offers a veryadvantageous way to increase the capacity of the compressed air systemeither when a new site of use is added to the system, in which thecompressed air reduced in pressure can be recovered, or if the systemalready comprises such sites of use, from which recovery can be arrangedin a simple manner. If the capacity of a conventional open compressedair system is nearly entirely in use, very large investments arepossibly required to increase the production and after-treatmentcapacity of the compressors. Such an expensive investment can be avoidedby using the solution of FIG. 2, because the capacity of the basicsystem 20 then need not be increased, if the drives 25 are separatedfrom the system into their own closed cycles or new drives 25 are addedto it, since these drives 25, which are in a closed cycle, do not at allincrease the amount of air needed by the basic system 20. This way, anexpensive capacity increase of the basic system can be replaced by aninexpensive additional compressor 21 that does not even need anyafter-treatment equipment.

If we examine generally a compressed air system of the invention thatrequires after-treated compressed air (maximum dew point requirement isfor instance +2° C., −20° C. or −40° C.) and the compressed airproduction system comprises oil-lubricated screw or rotary compressors,the after-treatment equipment is in practice not needed and thus doesnot cause any pressure loss, so the energy saving is approximately 15%.In addition, it is possible to use nearly isothermal compression, whichmeans an energy saving of 25%. If there is a 2-bar pressure in thesuction pipe (=return pipe) and the delivery pressure of the compressoris 9 bars to produce a 7-bar pressure for the site of use, an energysaving of over 15% is achieved. If all above-mentioned savings can beincluded in the same system, the total saving is over 50%. With othercompressor types, the energy saving is approximately 25%, because theircompression process cannot be improved in the same manner as that ofoil-lubricated screw and rotary compressors. In this case, too, theafter-treatment system is unnecessary.

If no requirements are set on the dew point and the compressors areoil-lubricated screw or rotary compressors, the same result as above isachieved. In this case, too, dried suction air should be used so as toachieve a nearly isothermal compression process in the compressor. Theenergy saving is the same as above. When using other compressor types,dried air does not need to be used in circulation, and moist air is alsosuitable. Removing condensed water from such replacement air can be donein an elementary manner, for instance by means of conventionally builtwater reducers, if desired and necessary. Energy saving is thenapproximately 25%.

It should also be noted that circulation can be done in any compressedair system on some level by using either an existing compressor or bytaking into use a new compressor (or compressors) dedicated tocirculation. The possibilities and the extent to which the invention canbe applied are determined according to the structure and type of thesystem.

The system of the invention for producing and distributing compressedair is described above using only some exemplary embodiments and it isto be understood that these systems can be modified without departingfrom the scope of protection defined in the attached claims.

1. A system for producing and distributing compressed air, comprising: asource of compressed air; at least one compressor having connectedthereto a suction pipe for the intake of air from the source ofcompressed air and an output pipe for air compressed by said at leastone compressor; distribution piping connected to the output pipe fordistributing air to sites of use; a return pipe between the suction pipeand at least one site of use for receiving air reduced in pressure in itand feeding it back to said at least one compressor; equipment betweenthe suction pipe and the source of compressed air for controlling theintake of air from the source of compressed air; and means for bypassingsaid at least one compressor.
 2. A system as claimed in claim 1, whereinsaid equipment comprises means for setting the pressure prevailing inthe suction pipe of said at least one compressor, and wherein the systemfurther comprises means for opening a connection from the return pipe toatmosphere and means for separating the return pipe from the suctionpipe.
 3. A system for producing and distributing compressed air,comprising in combination: a distribution system for distributingcompressed air; and a branch system connected to said distributionsystem by a flow control device that controls intake of compressed airfrom said distribution system during operation of said branch system,said branch system not providing air back into said distribution system,said branch system comprising a compressor having connected thereto asuction pipe for intake of air from said flow control device and anoutput pipe for air compressed by said compressor, means for bypassingsaid compressor, piping connected to said output pipe for distributingair to sites of use, and a return pipe between said suction pipe and atleast one of said sites of use for receiving air reduced in pressure andfeeding the reduced pressure air back to said compressor.
 4. The systemof claim 3, wherein said flow control device comprises a flow meter thatmeasures an amount of air taken from said distribution system.
 5. Thesystem of claim 3, wherein said flow control device comprises means forsetting a pressure prevailing in said suction pipe of said compressor,and wherein said branch system further comprises means for opening aconnection from said return pipe to atmosphere and means for separatingsaid return pipe from said suction pipe.
 6. The system of claim 3,wherein said branch system does not include an air dryer.